Matthew 19:9

I say unto you. See on ch. 5:22. The only change made to accommodate the original marriage law to a fallen world is that violation of the marriage contract by unchastity may constitute a lawful basis for dissolving the union. Otherwise, the union may not lawfully be broken.

Whosoever. The principle Christ is about to state is of universal application. No one who professes to be a Christian should think himself an exception to it.

Fornication. Gr. pomeia (see on ch. 5:32). It should be noted that in the NT the term “fornication” covers all illicit relationships both before and after marriage. To the modern English reader the word “unchastity” conveys more exactly the meaning of pomeia as it is used in the NT. Under the Mosaic law the penalty for marital unfaithfulness was death (see on Lev. 20:10), not divorce. Furthermore, under Moses’ law the death penalty was mandatory, whereas under the Christian law here set forth by Christ divorce is not mandatory, but permissible. From Jesus’ teachings here it may be inferred that the innocent party is free to choose whether the marriage relationship shall be continued. Reconciliation is ever the ideal, especially if children are involved.

Here and in Jesus’ parallel discussion in Matt. 5:32 it seems to be implied, even though not specifically stated, that the innocent party to a divorce is at liberty to marry again. This has been the understanding of the great majority of commentators through the years.

Marrieth her which is put away. Any alliance she contracts with another man violates her original marriage vow, which violation constitutes adultery. By a parity of reasoning the man marrying her becomes an adulterer also.